Visual indicators are often used in small electronic appliances, such as razors, modems, computers, audio and video equipment etc, to indicate that the appliance is powered ON or is in some given state. The visual indicator, which the user sees on the appliance, is generally of small size and of simple circular or square geometry. Often the appliances are powered by batteries for ease of use or to minimize cost or power requirements; thus, small low power drawing sources of illumination are used, such as light emitting diodes (LEDs). However, given the small size, illumination characteristics and illumination power, these light sources require enhancement to meet visual and aesthetic requirements for the appliance. Thus, to enhance these visual indicators the appliances use light diffusing apparatuses with the light source.
The visible surface of the light diffusing apparatus is usually located flush with the surface of the appliance enclosure. The light diffusing apparatus can be either attached to the enclosure or to a printed circuit board (PCB) along with the illumination source, typically a light emitting diode (LED). The apparatus is generally made of optically clear polymers (acrylic, polycarbonate, etc) and is shaped in such a way that the output of the LED is distributed uniformly across the visible exit surface which is grounded to enhance visibility.
A light pipe is a commonly used light diffusing apparatus which provides significant manufacturing advantages. Light pipes economically adapt the emission pattern of a standard off-the-shelf LED. LEDs usually have a radiation pattern of circular or elliptical symmetry with a maximum intensity on the optical axis (an axis which is usually normal to the exit window of the LED) which decays to zero at a direction of 90 degrees from the axis of illumination. A light pipe adapts the emission pattern of an LED to the esthetic requirement of a visual indicator by guiding the LEDs output to create a uniform illumination across the entire surface of the visual indicator. Light pipes also increase the area over which the light from the LED is spread allowing the visual indicator to be significantly larger than the LED which provides the illumination. Light pipes offer other advantages in that they provide a link between the LED, preferably soldered to the PCB, and the indicator position on the surface of the appliance's enclosure. The light pipe also facilitates the appliance's assembly and, in the case where it is attached to the enclosure, no mechanical contact to the PCB is required.
In many instances, it is necessary to have an indicator which can vary the color it displays. One standard method involves using an off-the-shelf dual color LED (two LEDs mounted in a single package) and fitted with a glass filled clear epoxy lens. The exit surface illumination and color uniformity is good. However, there are many limitations. The glass filled epoxy is a bulk diffuser i.e. light is diffused in all directions through the solid and is visible from all surfaces, thus lowering illumination efficiency. The LED has to be soldered to the PCB, which limits the packaging flexibility, and the indicator shape which the user of the appliance sees at the surface of the appliance is limited to the standard catalog shapes (usually square, rectangular or round). Moreover, there are no dual color LED packages with large-size diffusing lenses, that can extend through enclosures walls for instance, which are available in Surface Mount Technology (SMT) packages (with no leads).
SMT LED packages are very small, varying in size from a fraction of a mm up to about 6 mm for each side. The distance between the optical axis of two individually packaged SMT LEDs mounted side by side is about 0.5 mm to 1 mm. For two or three LED's mounted in the same package, the distance between their optical axis is around 0.7 mm to 0.8 mm. The most common two LEDs packages are bicolor (some packages have two LEDs of the same color), with orange, red and various tints of green being the most common colors. In the case of a red and green bicolor LED, varying the ratios of red and green creates a color, where the LED beams overlap, with a tint which can be continuously adjusted from green to red with the intermediate tints of yellowish green, yellow, amber, and orange. Three LED packages are usually tricolor with one of each LED of red, green, and blue color (blue LEDs devices are currently significantly more expensive than red and green devices); mixing all colors results in a white output and a partial mix of the three colors can generate almost any visible spectrum color. One obvious market for the tricolor LEDs is imaging displays where the representation of pictures or video content requires a full color capability. The color mixing capabilities of the light pipe then becomes a critical performance factor. However, even with such small distances between the optical axis as the ones described previously, good overlap is not obtained with most commercially available light pipes, which were designed for the more common single color LED packages, and multiple colors can be viewed on the exit surface instead of one solid uniform color.
Given the current state of the art, light pipes used for single color indicators on most small electronic appliances have a small visible exit surface. In most cases, the exit surface 15 is located at the end of a relatively long section 14 of FIG. 1. The long section 14 is required to homogenize the LED output and the relative size of the exit surface 15 is small compared to the length of the mixing section 14. Another example with similar characteristics is U.S. Pat. No. 5,581,683 granted to Bertignoll et al on Dec. 3, 1996, which contains a description of a uniform light pipe with an exit surface of 3.3 mm.times.51.5 mm; illumination is provided by two LEDs 16 located at the end of a U-shaped plate 50 mm away from the exit surface.
The example of the preceding paragraph both use Total Internal Reflection (TIR) as the mechanism to bend the light rays emitted from the source in order to obtain uniform illumination at the light pipe exit surface. TIR, a well know scientific principle, will be present whenever a light ray strikes a surface at an angle less than a critical angle, measured relative to a tangent at the surface encountered by the light ray, when going from a high refractive index medium to a low refractive index medium, in this case from the light pipe polymer to air. If the angle is larger than the critical angle with respect to a tangent at the point of the surface the light ray strikes, the light ray will be refracted and will escape from the light pipe. However, if the light ray strikes at a smaller or shallower angle than the critical angle it will be reflected. Thus, in constructing a light pipe based on TIR it is thus important to eliminate all abrupt angular changes on the light pipe structure, which factor results in the long structures of the prior art.
A large area linear light pipe is proposed by Simms in U.S. Pat. No. 5,590,945. The device is essentially a flat plate illuminated from its end and which has an accurate rear reflecting surface covered with triangular reflecting ridges which deflect the light toward the front surface. It is said that the design can also be used with multicolored light sources such as bicolor or tricolor LEDs. However, this apparatus cannot be scaled down because the triangular ridges become almost impossible to manufacture with precision, in a common plastic injection process, when features dimensions need to be 0.5 mm and smaller. Such small ridges would be required for a device where deflection has to be performed within a distance of about 20 mm or less. Also, in this apparatus, light mixing is favored by the large dimensions of the structure, which is designed for highlighting an automobile dashboard or console; a small scale version of the apparatus would show non uniform color mixing. Finally, in many electronic appliances, it is preferable to locate the LED substantially under the light pipe whereas this apparatus is designed for the packaging constraints of an automobile dashboard.
Both a uniform large viewing area and efficient color mixing is obtained with apparatuses described by Koike in U.S. Pat. No. 5,542,017. These apparatuses use nonuniform bulk scattering where scattering is progressively higher as the light rays are farther from the light source. The applications are primarily for thin plate illumination devices used for Liquid Crystal Display backlighting. Illumination is from the edge and a mirror is proposed on the plate bottom to enhance brightness. Fabrication of the nonlinear bulk scattering media is most effective in simple geometry diffusers, especially if a mirror is added to enhance brightness; the cost can easily become prohibitive when the light pipe complexity increases, in non-symmetric three-dimensional shapes for example, and for appliances which are offered in a highly cost-competitive market.
Another approach proposed by Anderson in U.S. Pat. No. 5,414,598 for large area illumination and LCD display backlighting is to use an imbedded beam-splitter to divide the spot light into multiple sub-beams. Two sub-beams are created by reflection on a triangular opening (hole) within the light pipe, which has its apex aligned to the light source optical axis, and the remaining of the beam continues directly into the structure. All reflections are created by TIR. The three sub-beams are then redirected into a large flat plate type area where the bottom surface is light diffusing. The apparatus does not provide a color mixing mechanism and it would be very sensitive to the alignment of the multiple light sources relative to the triangular beam-splitter apex. In fact one of the claimed advantages of this invention is that it only uses one light source. Additionally, as depicted in FIG. 2, of the light pipe of Anderson, the light from the light source 17 must pass through a long convoluted chamber before reaching the exit surface 19.
Thus, there is a requirement for a compact light pipe which would not only provide a uniform illumination over its exit surface but would also exhibit good color uniformity in the case where multiple light sources of various colors are used simultaneously. A light pipe or light diffusing and mixing structure which also eliminates the need for a long and convoluted dispersing and mixing channel. A structure where the light source can be positioned so that it faces the exit surface and is a short distance from the exit surface.
Light pipes, such as those described above and the ones described herein, are often used on consumer products. In particular, they are used in small hand-held consumer products such as electric razors, hand-held recorders, cordless or cellular telephones and similar devices. The light pipes are often used to provide backlighting or to indicate some state that the device is in at the particular time, such as a recording state for a hand-held recorder or a battery charging or discharge state, etc. The light illumination devices, or light sources used with light pipes are generally LEDs, and are usually powered by the same batteries which power the hand-held consumer device. In particular, two common and popular batteries used in such devices are nickel cadmium (Ni-CAD) and nickel metal hydride (Ni-MH). Often, it is desirable and in some instances a practical necessity to indicate the state of charge of the battery which powers the device. The obvious choice for indication of the state of charge of a battery is in many instances a light pipe with an appropriate visual display and an LED which provides the light source of illumination of the light pipe.
In systems currently in use monitoring is often performed using a programmed electronic micro-controller or dedicated application specific integrated circuit (ASIC). Mathematical integration forms the basis of the monitoring scheme. The monitoring schemes quite often measure time spent in discharging the battery and subtract that from the time spent in the charging mode. Quite often, the discharge time and charging time are first multiplied by appropriate weighting factors before the subtraction is performed. These particular weighting factors are often derived from assumptions on the current magnitude drawn from or delivered to the battery in each of the modes. In the most elaborate schemes, the actual current, delivered or drawn, is measured to determine the weighting factors.
However, the implementation of these schemes is generally quite costly and thus only economically feasible for expensive hand-held consumer products. Often, for less expensive models, a voltage based monitoring scheme of the battery is used. However, in order for the battery to provide useful power the battery must provide an almost constant voltage when discharging. In fact, battery manufacturers strive to provide batteries which discharge at a constant voltage. However, production of a battery with a perfectly constant voltage is nearly impossible. Thus, in reality, the discharging profile of a fully charged Ni-CAD or Ni-MH battery will, with a constant load, have the following profile: (a) first, the voltage decreases rapidly, (b) then the steep decrease subsides and the voltage falls very slowly for most of the discharge cycle of the battery, and (c) at the end of the discharge cycle, the voltage again starts to decrease rapidly again.
Thus, in reality, these voltage-based schemes can only provide a fairly reliable indication of charge at the beginning of the discharge cycle when the battery is fully or nearly fully charged, and at the end of the discharge cycle when the battery is completely or almost completely drained. Consequently, most implementations of a voltage-based indicator only provide a low battery indicator.
Similarly, monitoring the voltage of a battery while it is being charged has the same or similar problems. Namely, it is easy to determine when the battery is fully discharged at the beginning of the charging process and to also determine with reasonable certainty that the battery has been fully charged at the end of the charging process However, it is difficult to monitor the progress of the charging of the battery during the long intermediate portion of the charging process.
Thus, what is needed is a fast and efficient voltage-based scheme to monitor the state of charge of a battery or batteries during the charging and discharging states. An inexpensive scheme which will provide a reasonably accurate indication of the state of the battery, whether in the charging or discharging state, and which will not draw too much power from the battery is desired.